Field of the Invention
The present invention relates to an optical waveguiding part comprising an output facet through which light is arranged to travel out from the waveguiding part. In particular, the invention relates to such a waveguiding part providing low reflectivity at the facet, of light travelling along the waveguiding part. For instance, the invention is useful in the case of a semiconductor laser component connected to such a waveguiding part, in particular when light is output through said facet via a modulator.
In many semiconductor optics applications, light traveling through a waveguide needs to be output via a facet, such as to air or to another waveguide or another light-carrying medium. This is in particular the case when coupling an output waveguide from a semiconductor laser to another component, such as a modulating component, or to an optical fiber, possibly via air. One important example of this situation is a laser transmitter component, such as an SFP+ (enhanced Small Formfactor Pluggable) component.
Description of the Related Art
Such facets in general give rise to loss and unwanted reflections. Such reflections are often harmful to the performance characteristics of the laser component.
In order to mitigate this problem, various anti-reflection coatings can be used. Also, the waveguide can be disposed in a direction that is not perpendicular to the facet, whereby the reflected light is directed away from the waveguide direction upon reflection.
However, this approach does not always provide adequate reflection management. For example, in some cases there are geometrical restrictions on the output beam direction for an optical circuit, so that only a modest angle of the waveguide relative to the facet normal inside the chip is permissible, and in this case the associated reduction of modal reflectance may be insufficient. This situation may occur even for a weak shallow rib waveguide, but it is more likely to occur with a deeply etched waveguide that has strong lateral light confinement.
Furthermore, it has been proposed to use a so-called “window” region near the facet. The window region is created by first etching away the waveguide near the facet, and then filling the etched region with transparent material. This allows the light to diffract vertically and horizontally prior to hitting the facet in question, thereby achieving a reduction in the modal facet reflectivity.